Ethanol is widely used commercially as a gasoline additive or as a fuel per se, as a solvent, as a germicide, as an antifreeze, as a component in the food and beverage industry, and as a chemical feedstock. It is particularly useful as a feedstock for acid-catalysed reactions such as the dehydration to ethylene. See e.g. WO 2008/138775 which discloses a process for the dehydration of one or more alcohols comprising contacting the one or more alcohols with a supported heteropolyacid catalyst in the presence of one or more ethers; and WO 2008/062157 which discloses a heteropolyacid catalyst and the use thereof in a process for the production of olefins from oxygenates.
Ethanol is of increasing significance as a chemical feedstock, since it is readily obtainable from biological sources, in particular by the fermentation of sugars and/or biomass. Ethanol from biological sources, so-called bio-ethanol, thus provides one way of reducing the dependence on crude oils for fuel uses and as chemical feedstocks.
Ethanol, particularly bio-ethanol (or ethanol obtained by fermentation) typically contains low levels of nitrogen-containing contaminants. One possible source of nitrogen-containing contaminants may be ammonia which may be introduced during the fermentation stage. Once in the process, the ammonia can react with ethanol and other impurities to form a variety of nitrogen-containing compounds.
The presence of nitrogen-containing contaminants in ethanol is undesirable since these compounds may interfere with subsequent chemical processing in which the ethanol is used as a feedstock. For example, nitrogen-containing contaminants, which may be volatile nitrogen compounds such as acetonitrile and ammonia, and particularly acetonitrile, can poison, deactivate or otherwise interfere (e.g. act as a precursor to a catalyst poison) with a number of catalysts which may be used in the processing of alcohol feedstocks, for example by neutralising acidic sites on heterogeneous acidic catalysts. This may lead to a loss of process efficiency and a need to undesirably replace the catalyst more frequently. Approaches have been taken to reduce the level of acetonitrile in ethanol feedstocks, with such approaches including aqueous extraction, sacrificing acid and adsorption.
WO 1997/045392 discloses a process for the production of ethers in which deactivation of an acidic ion-exchange resin etherification catalyst is reduced by separating nitriles from an olefin feedstock by aqueous extraction. The nitriles are subsequently separated into an alcohol phase and hydrogenated to form amines which are more easily separable from the alcohol phase by fractionation.
EP 1 176 132 A1 discloses a process for preparing ethers comprising reacting an alcohol and an olefin in the presence of an acidic catalyst. Excess alcohol is recycled to the reaction zone together with nitrile compounds originating from the olefin feed. To avoid accumulation of nitriles in the system and deactivation of the catalyst, the excess alcohol comprising nitrile compounds is contacted in the liquid phase with a solid acid prior to being recycled to the reaction zone. It is reported that this reduces the level of nitriles in the recycled alcohol stream by at least 50%.
WO 2010/060981 discloses a process for the purification of an alcohol in the course of a process for the preparation of olefins by acid-catalysed dehydration of the alcohol, the process comprising contacting the alcohol with one or more adsorbent materials. It is disclosed in WO 2010/060981 that while ammonia and amines can be adsorbed, nitrile impurities such as acetonitriles must be hydrogenated to provide modified impurities which are more readily adsorbed. Thus, according to WO 2010/060981, the alcohol feed is subjected to a hydrogenation step prior to contacting the alcohol with the one or more adsorbent materials. The Examples of WO 2010/060981 teach the removal of basic compounds from bio-ethanol by adsorption on a sulfonic acid resin at ambient temperature and pressure.
However, such means to reduce the level of acetonitrile are relatively inefficient (e.g. requiring additional process steps or relatively poor reduction in acetonitrile level) and lead to other disadvantages such as a need to dispose of the aqueous extract or the need to replace the material onto which contaminants are adsorbed.
Accordingly, there remains a need for a means to remove nitrogen-containing contaminants including volatile nitrogen compounds (especially acetonitrile) from an ethanol feedstock while avoiding the drawbacks of the prior approaches, such as the aforementioned inefficiencies and need to replace the means being employed, to reduce the level of volatile nitrogen compounds in an ethanol feedstock.